Method of heat treating iron



Patented Oct. 15, 1929.

UNITED STATES,

PATENT OFFICE AUGUSTUS F. MEEHAN, OF CHATTANOOGA, TENNESSEE, ASSIGNOR TO MEEHANITE METAL CORPORATION, OF CHATTANOOGA, TENNESSEE, A CORIOEATION OF TEN- NESSEE METHOD OF HEAT TREATING IRON No Drawing. Continuation of application Serial No. 284,677, filed. 'June 11, 1928. This application filed July 22, 1929. Serial No. 380,257.

The present invention relates to the manufacture of castings from iron, and particularly castings made from nearly or wholly white iron. An object of the invention is to provide a new and improved method for heat treating cast iron whereby improved physical properties are obtained in a shorter time and more economically than has heretofore been possible, these results being obtainable primarily by reason of the character of the iron treated.

The invention is based upon the discovery that alkaline earth metals (magnesium, strontium, ctc.,) in metallic form, or as a silicide, which preferably is substantially free from iron, and particularly metallic calcium or calcium silicide, have a marked effect in facilitating what it is desired should take place during the heat treatment operations.-

In heat treating iron for the purpose of making a ductile product like malleable iron, it has long been recognized that the combined carbon cannot be entirely decomposed or broken up (dissolved-inthc solid solution) above the critical range. In other Words, all but approximately nine-tenths of one per cent of the combined carbon content is ordinarily broken up if the casting is kept for a suitable period at a temperature above the critical. Furthermore, it has long been known that by allowing the casting to cool slowly through the critical temperature, this remaining amount will finally be precipitated, leaving substantially no combined car- .bon in the finished product.

In ordinary malleable iron practice, the iron castings from the air furnace areusually packed in carefully sealed iron pots, which are charged into annealing ovens where they are brought from a temperature from 1550 F. to 1625 F., at which they are held for from to hours. After this period, the casting in the oven must be cooled slowly (not over 10 per hour) until their temperature drops to about 1250 F. This process requires on an average of about seven days.

The present invention by reason of the specially prepared product to be heat treated and the newly discovered response of the same under heat treatmentcontemplates a considerable reduction in time required for these heat treatments, and an improvement in the results obtained, more particularly in the formation of graphite carbon and in the physical properties. I v

p More specifically, the invention relates particularly to the treatment of specially treated'white orslightly gray cast iron, as distinguished from what is known in the trade as ordinary gray iron. It'has been discovered that this specially prepared product when heat treated produces results heretofore unexpected anda product of a radically different nature from anything previously obtained by heat treatment of other products.

An important feature of the invention is the treatment of the molten white iron with calcium, either in the form of calcium silicide or as metallic calcium, since it has been discovered that the calcium addition imparts a pronounced .efiect to the heat treatment.

The heat treatments will, of course, vary depending upon the. result desired. In order that the' invention may be fully understood, I will describe the same in connection with an annealing heat treatment. The heat treatments of this type, as I have employed them, have varied somewhat, depending upon whether the iron to be treated is cupola iron or air-furnace iron, although in both instances, of course, the ironis the specially prepared product treated with an alkaline earth metal.

Referring first to the heat treatment of cupola iron as it comes to the furnace the same will be treated with calcium, preferably with an amount insuflicient to turn the fracture gray, but always in amount in excess of that which would be neutralized by any element in the molten mixture.

Since the character of the molten iron will vary in its characteristics, the amount of of alkaline earth metal to be used must be governed accordingly. This is explained in in Patents 1,683,086 and 1,683,087, granted Sept. 4, 1928. The alkaline earth metal may be introduced into the molten metal in any suitable manner, either separately or in combination with other ingredients, i. e., as an alloy. Ordinarily, the alloy or separate ele- I or ments are reduced to relatively small particles, although they may be used 1n brick or briquette l' OI'III. The agent may be added in the spout as the molten metal runs from the 5 furnace or thrown into the stream of metal where it enters the ladle from the spout, or else added to the metal in the furnace.

The metallic calcium treated white iron will have substantially the following analysis:

Silicon .90 to 1.20 Sulphur .09 to .13 Phosphorous .18 to .30 Manganese .20 to .50 Total carbon 2.70 to 3.20

Other irons which have been treated have been of approximately the following analyses As will be noted, this iron contains the manganese as a neutralizing agent for the sulphur, and therefore only a comparatwely small amount of alkaline earth metal, such as calcium or calcium silicide, need be employed, since virtually all added will be in excess of that which would be neutralized by any element in the molten mixture. Obviously, in the absence of manganese, or other sulphur neutralizing agent, a greater amount of alkaline earth metal would be required in order to have present an amount in excess of that which would be neutralized.

-The physicalcharacteristics of this metal will be substantially as follows: Tensile strength..;.. 50,000 to 60,000 pounds I per square inch. Elongation Nil. Reduction of areagm Nil.

Brinell hardness 320 to 360.

Fracture Pearl colored. Machineability Too hard to machine. Transverse strength- 3,000 to.4,000 pounds.

Tensile strength 80,000 to 90,000 pounds per square inch.

Elongation 1% to 1 Reduction of area- 1% to 2%.

Brinell hardness 220 to 240.

Machineability Fair. Transverse strength- 6,000 to 8,000. Fracture Mottled gray. Deflection a 44" to f g.

placed in a furnace and heated softness, a slightly varied heat treatment is followed. In this case, the castings are as quickly as possible to 1650 F., at which temperature they are held from 20 to 25 hours. The castings are then allowed to cool very slowly,

preferably at a rate not in excess of 10 per hour, until the furnace has reached a temperature of approximately 1000 F. The castings are then removed and are allowed to cool at room temperature. This heat treatment results in the following physical properties:

Tensile strength 45,000 to 55,000 pounds per square inch.

Elongation 4% to 6%.

Reduction of area--- 5% to 7%.

Brinell hardness 120 to 150.

Fracture Blackinbacl;-gr0und slightly mottled.

Machineability Good.

Transverse strength- 4,500 to 5,000.

Deflection; A to '7 Tensile strength; 24,700 pounds per square inch.

Brinell 402.

Fracture White.

The same white iron not treated with calcium and given the first (1) heat treatment.

showed the following characteristics:

Tensile strength 43,800 pounds per square 1IlCll. Brinell number 394. Fracture White.

The same metal without the calcium, and

given the second (2) heat treatment showed the following: Tensile strength 48,700 pounds per square mch. Brinell 202. Fracture White with back-ground ,of large graphite flakes.

In neither case was there. an elongation or reduction of area apparent.

The calcium treated metal given the first (1) treatment showed the following characteristics Tensile strength-.. 62,100 pounds per square inch. Brinell number 302.

Fracture Slightly mottled.

. \I Q Given the second (2) treatment, the metal It has also been found that the calcium employed in the form of calcium silicide will materially effect the heat treatment.

The calcium silicide treated white. metal shows the following characteristics,when not given the first (1) heat treatment:

Tensile strength" 29,700 pounds per square inch. Brinell 402. Fracture White.

\Vhen this calcium silicide metal was given heat treatment (1) 1t showed:

Tensile strength 83,700 pounds per square- 111C 1. Brinell number 212. 23 Fracture Light gray.

When given the second (2) treatment,the calcium silicide treated metal shows:

Tensile strength; 44,800 pounds per square 39 inch.

Elongation 7.8. Brinell number 95. Fracture Black.

Thus it is apparent that metal not treated with calcium either in metallic form or as calcium silicide,although showing an increased tensile strength after heat treatment, does not show a tensile strength as high as 40 the same metal which. has been treated with the calcum either in metallic or silicide form- Furthermore, the results show that there is no elongation or reduction of area obtained by either heat treatment, unless the calcium or calcium silicide is nsed,,and then with the second (2) heat treatment the metal shows a marked elongation. In other words, with these heat treatments, unless the metal is treated with calcium in some form, the same strength cannot be secured nor can the elongation be obtained.

Additionally, it has been found that there is a marked difference in the state of the carbon. The white iron without the addition of calcium, either as a silicide or in'metallic form, showed:

Total carbon 2.86

Combined carbon 1.00

The same metal treated with metallic-cal cium shows:

- Total carbon 2.58

After heat treatment #1:

Graphitic carbon--. 1.65 Combined carbon .01

The same metal after heat-treatment'#2:

Graphitic carbon 2.25 Combined carbon .35

The same metal treated with calcium-silicide:

The calcium, therefore, facilitates the heat treatment by permitting the carbon to pre- '90 cipit-ate and secure a higher percentage of graphitic carbon with a lower percentage. of

combined carbon.

The same results are obtained in the heat treatment of air furnace iron, providing the as calcium is added either-as metallic calcium or as calcium silicide. The air furnace iron is of the following approximate analysis:

Silicon .80 to 1.00% Sulphur .05 to .06 Phosphorous .15 to .20 Manganese .20 to .30 Total carbon s 2.00 to 2.50

The physical characteristics of the metal '105 are:

Transverse strength" O' 0 5,000 pomlds- 5 This metalis treated with calcium either as a silicide or in metallic form, in an "amount to be in excess of that which would be neutralized by any element in the molten mixture and preferably in an amount insufficient to 1:0

turn the fracture gray. As explained above. the particular molten metal here discussed j contains man anese which serves as a neutralizing agent f br the sulphur, and therefore only a small amount of alkaline earth metal need be employed, since virtually all of the amount employed would be in excess of that which would be neutralized by any element in I 1 the mixture. Of course, in the absence of other neutralizing agents for sulphur, or any other element in the mixture, it would be necessary to use a greater amount of calcium or other alkaline earth metal. may be given either of the following heat treatments:

(1). The castings areplaced in a furnace and heated as quickly as possible to 1650 F.

7 At this temperature they are held for at least approximately 16 hours, and then immediatelywithdrawn and allowed to cool at room temperature. The physical characteristics of castings thus treated are:

Tensile strength 90,000 to 110,000 pounds per square inch.

' Elongation .1

Reduction of area 1 Brinell hardness 200 to 230.

Fracture Mottled. Machineahility Fair. Transverse strength 6,000 to 8,000 pounds. Deflection; A to If softer and more ductile castings are desired at the sacrifice of some tensile and transverse strength, the following heat treatment (2) is employed.

(2) The castings are placed in a furnace and heated'as'quickly as possible to 1050 F. This temperature is maintained for at least approximately 16 hours when .the castings are allowed to cool slowly in vthe furnace,

7 preferably at a rate not exceeding 10 per hour to 1000? F., and thereafterthe castingsare allowed to cool at room temperature. This slightly Varied heat treatment for calcium treated cupo'la iron produces the following physical characteristics:

Tensile strength 55,000 to 65,000 pounds per square inch.

It will be observed that the heat treatments (1) and (2) for calcium containing cupola iron vary only slightly from the'heat treatments of calcium treated air furnace'iron, the prin'ci pal difference being the period for which the castings are maintained above the critical temperature. Calcium treated .cupola iron is maintained at 1650 F for from 20 to 25 hours, whereas the cal ium treated air furnace iron is maintained above the critical temperature for approximately 16 hours.

, Both the air furnace and cupola irons may be given either the heat treatment (1) or the heat treatment (2). In the heat treatment (1) the castings are immediately removed from the furnaceand permitted to cool at room temperature. In heat treatment (2) the castings are allowed to cool slowly in the fur-' The metal then nace at a rate not exceeding by much 10 per. hour, to 1000- F.,-and after thus passing treated with results of an unexpected and remarkable nature. I have herein disclosed a number of heat treatments merely as ex amples, and it will be understood, of course,

that the heat treatments may be varied without departing from the invention, since the particular heat treatment employed in each case will depend uponthe particular result desired. A more specific aspect of the invention involves the discovery that when a heat treatment is employed a smaller amount of alkaline earth metal than is necessary to turn the fracture gray may be used Withvery satisfactory results.

By calcium as used in the specification and claims. is meant either metallic calcium orcalcium silicider. By white iron? is meant such castings as are substantially free from graph'itic carbon. By gray iron is meant castings in which more or less graphite is present. By molten white iron is meant such molten iron as will produce castings substantially free from graphitie carbon, and by molten gray iron is ineant such molten iron as will produce castings containing more or less graphitic carbon.

Obviously, the process described. in detail may be varied considerably without departing from the invention, the essential features of which are set forth in the following claims.

This application is a continuation of myco-pending application Serial No. 284,677, filed June 11, 1928.

I claim:

1. In the process of manufacturing cast iron, the improvement which consists in heat treating a casting resulting from molten metal to which has been added an alkaline earth metal in an amount in excess of that which would be neutralized by any element of the molten mixture.

2. In the process of manufacturing cast iron, the improvement which consists in heat treating a casting resulting from molten metal to which has been added an alkaline earth metal in an amount in excess of that which would be neutralized by any element of the molten mixture, but in an amount insufficient to turn the fracture gray.

3. In the process of manufacturing cast iron, the improvement whig1 consists in heat treating a casting resulting from molten metal to which has been added calcium in an amount in excess of that which would be neutralized by any element of the molten mixture.

4. In the process of manufacturing cast iron, the improvement which consists in heat treating a casting resulting from molten metal to which has been added calcium in an amount in excess of that which would be neutralized by any element of the molten mixture, but in an amount insuificient to turn the fracture gray.

5. In the process of manufacturing cast iron, the improvement which consists in heat treating a casting resulting from molten metal to which has been added calcium silicide in an amount inexcess of that which would benutralized by any element of the molten mixture.

6. In the process of manufacturing cast iron, the improvement which consists in heat treating a casting resulting from molten metal to which has been added calcium silicide in an amount in excess of that which would be neutralized by any element of the molten mix- I ture, but in an amount insuflicient to turn the fracture gray.

7 In the process of manufacturing cast iron, the improvement which consists in heat treating by maintaining at a temperature above the critical for a period in excess of twelve hours and then cooling as desired a casting resulting from molten metal to which has been added an alkaline earth metal in an amount in excess of that which would be neutralized by any element of the molten mixture; l

8. In the process of manufacturing cast iron, the improvement which consists in heat treatin by maintaining at a temperature above the critical for a period in excess of twelve hours and then cooling as desired a casting resulting from molten metal to which has been added an alkaline earth'metal in an amount in excess of thatwhich would be neutralized-by any element ofthe molten mixture, but in anamount insufficient to turn the fracture gray.

In testimony m hand. y AUGUSTUS F. MEEHAN.

whereof have hereunto set 

